1. Field of the Invention
The present invention relates to an epitaxial wafer and a compound semiconductor light emitting device, and methods of fabricating the same, and more particularly, it relates to an InGaN-based epitaxial wafer and compound semiconductor light emitting device employing volatile compound semiconductor substrates of GaAs, GaP, InAs or InP, and methods of fabricating the same.
2. Description of the Background Art
FIG. 4 is a sectional view showing the structure of a GaN-based blue or green light emitting device (LED) employing a sapphire substrate described in Nikkei Science, October 1994, p. 44, for example, which is now being put on the market.
Referring to FIG. 4, a clad layer 14 consisting of n-type AlGaN, a luminescent layer 15 consisting of InGaN, a clad layer 16 consisting of p-type AlGaN and a GaN epitaxial layer 17 are successively formed on an epitaxial wafer which is formed by a sapphire substrate 11, a gallium nitride (GaN) buffer layer 12 which is formed on the substrate 11 and a hexagonal n-type GaN epitaxial layer 13 which is formed on the GaN buffer layer 12 in this blue or green light emitting device. Ohmic electrodes 18 and 19 are formed on the GaN epitaxial layers 13 and 17 respectively. In this blue or green light emitting device, the GaN buffer layer 12 is adapted to relax distortion caused by the difference in lattice constant between the sapphire substrate 11 and the GaN epitaxial layer 13.
Referring to FIG. 4, this blue or green light emitting device employs insulating sapphire as the material for the substrate 11, and hence two electrodes must be formed on the same surface side in order to fabricate the device. Thus, patterning through photolithography must be performed at least twice while the nitride layers must be etched by reactive ion etching, whereby complicated steps become necessary. Further, sapphire is hard to handle in element isolation, due to its high hardness. Thus, employment of the sapphire substrate is problematic in process cost. In view of application to a light emitting device, sapphire cannot be applied to a laser diode having an optical resonator which is formed by a cleavage end plane, since the same is impossible to cleave.
Further, it has been difficult to fabricate a blue or green light emitting device by a conventional growth method, since the growth temperature is so high that growth with a high In composition ratio cannot be attained in the InGaN layer 15 which is an active layer. In addition, zinc (Zn) must indispensably be introduced as an emission center, leading to such technical problems in application to a device that the emission wavelength is widened and display performance of a full-color display is deteriorated.
To this end, it has been attempted to employ a volatile compound semiconductor such as conductive GaAs as the material for the substrate in place of sapphire having the aforementioned disadvantages. When the substrate is prepared from GaAs, however, an epitaxial wafer comparable to the wafer employing a sapphire substrate cannot be obtained under similar conditions.
Thus, a number of studies have been made in order to form GaN epitaxial layers on GaAs substrates. Most of these studies are adapted to form buffer layers consisting of GaN, AlN, GaAs and the like on GaAs substrates and thereafter form GaN epitaxial layers on the buffer layers.
However, the difference in lattice constants, or lattice incommensurateness between GaAs and that GaN and that between GaAs and AlN are 20% and 23% respectively.
If an epitaxial wafer which is fabricated by forming a buffer layer consisting of GaN or AlN on a GaAs substrate and further forming a GaN epitaxial layer thereon as described above is applied to a light emitting device (LED), therefore, there is a possibility that the life of the device is disadvantageously reduced by crystal defects.